C-STM: Circular Spatio-Temporal Modulation for Doubly-Spread Underwater Acoustic Channels

Underwater Acoustic Communication (UAC) is one of the main technologies for wireless underwater communications. However, UAC data transmissions encounter challenges posed by doubly spread channels in the complex and unpredictable underwater environment, leading to compromised overall reliability and robustness of UAC systems. Moreover, the Inter-Symbol Interference (ISI) and Inter-Channel Interference (ICI) make Multi-Input Multi-Output (MIMO) challenging. To address the challenges, a novel transmission method, named Circular Spatio-Temporal Modulation (C-STM), is proposed to utilize spatial diversity and improve the performance of UACs in doubly spread channels. The C-STM leverages the high auto-correlation of Zero-Correlation-Zone (ZCZ) sequences and the orthogonality of Inverse Digital Fourier Transform (IDFT) to defend communications against multipath delay and Doppler effects. To validate the efficacy of this proposal, several rounds of simulations were conducted based on real acoustic channel data collected from sea experiments. The results show that the proposed C-STM proves higher robustness against doubly spread channels compared to existing techniques, such as MIMO D-OFDM and PPM.